The invention describes a method and equipment for separation of solids from gaseous mixtures, especially for the separation of phthalic anhydride following its desublimation from a mixture with vapours of sublimable substances, steam, air or inert gases, which originate at sublimation refining or during production of phthalic anhydride by catalytic oxidation of naphthalene or o-xylene with air.
In Czechoslovak Pat. NO. 229768, corresponding to U.S. Pat. No. 4,528,006, a method is described together with an equipment for separation of phthalic anhydride from vapour-gaseous mixtures formed by oxidation of naphthalene or o-xylene with air. The mixtures are cooled from temperature in the region of 130.degree. to 250.degree. C. to the region of 50.degree. to 100.degree. C. by evaporation of water, which is sprayed into mixtures by nozzles in desublimation chamber. The openings of the nozzles are protected against the contact with the cooled vapour-gaseous mixture by washing with air, which is heated to the temperature of the input mixture.
This avoids the condensation of vapours and build up of solids on the nozzle openings. Simultaneously, air passes through gas permeable walls such as disclosed in U.S. Pat. No. 4,528,006, into a desublimator chamber. The temperature of the air is close to the temperature of the incomming gaseous mixture and it decreases later to a temperature which is lower than that of the mixture leaving the desublimator. Its velocity in the location of the entrance of the gaseous mixture is in the region of 10 to 1 cm/s depending upon the phthalic anhydride concentration in the mixture. It then gradually decreases to 1 to 0.2 cm/s. The velocity of vapour-gaseous mixture with desublimised solid products in the outlet from desublimizing space is in the region of 0.15 to 1 m/s.
The advantage of the method is the desublimation of bulks of mixtures without settling on the walls. Therefore, these walls need not be cleaned of a layer of solids. The several times higher throughput of the desublimation apparatus is a further advantage.
Vapour-gaseous mixtures which originate during the production of phthalic anhydride contain, however, tar like substances which at the separation of the product, stick to filter materials so that these filters must be periodically cleaned.
In addition to phthalic anhydride desublimizing in the form of wool flakes, which can hardly be transported from the apparatus, filtration equipment is complicated and explosion hazard is high at separation and transport.
These shortages are solved by a new continuous way of separation of solids especially of phthalic anhydride from vapour-gaseous mixture and by a new transport from the separator.